I'm not sure I understand the rules regarding sustaining spells. As I read it, if a mage casts a spell with a sustained duration, he suffers a -2 dice pool modifier for all tests. I believe that this may stack with each additional sustained spell.
Here's what I don't understand. The spell Combat Sense is a sustained spell. It provides a positive dice pool modifier to reactions. However, due to it being a sustained spell, the caster would have to have three successes to give a single net success to reactions (+3 from hits during casting, -2 from sustaining the spell, providing a net +1). Is that correct?